This invention relates to microporous filters and in particular to a self-cleaning filter of the type employing a replaceable cartridge. The cartridge is preferably of the form having a pleated structure consisting of a membrane folded back and forth between opposed boundaries, one of which serves as an inlet to material being filtered and the other as the filtrate output.
Microporous filters are defined as those filters with four sizes in the range of 0.02 to 10 .mu.m. In large volume applications employing microporous filters the operation is generally dead-ended, that is to say, all of the liquid presented to the filter inlet passes through a filter element which may be a membrane and resultant filtrate is withdrawn. In simple one pass flow through the filter, termed "dead-ended" flow, the particles, colloids or other macro-molecules contained within the liquid filter inlet material foul the filter cartridge until significant liquid flow ceases. The evidence of such cessation can be measured as a rise in inlet pressure. Such fouling occurs whether or not the microporous filters are of the flat filter construction or cartridge construction. The latter generally are cylindrically arranged wherein a shell containing a cylindrical filter cartridge defines together therewith an outer annular inlet chamber having the form of a cylindrical shell and an inner, centrally disposed chamber within the cartridge from which filtrate is withdrawn. Typically, such cartridge filters are of the pleated form in which the filter membrane is folded back and forth between cylindrically concentric boundaries, the outer one of which is located in the inlet chamber and the inner one of which is located in the outlet chamber.
In flat filter constructions tangential or crossflow systems operating with a inlet liquid being directed across the surface of the filter, have been found to improve clogging rates. A proportion (25-90% of the liquid flowing) flowing across the filter is either recycled or run to drain but in its path sweeps clogging particles and other foulants from the filter surface and thereby avoids clogging. In the use of microporous filters in filtration of blood fractions, a severalfold increase in throughput (i.e., volume of liquid that can be filtered before flow ceases or becomes uneconomic) occurs when liquid flow is arranged tangentially across the inlet surface rather than dead-ended.
It has now been found that cartridge filters can be operated in a tangential mode resulting in significant increases in throughput and reductions in filtration costs. Unexpectedly, cartridge filters of the type having a pleated construction which is folded between concentric cylindrical boundaries are found to respond to tangential flow operation even when such cartridges are disposed and operated in a filter construction in which the inlet side is generally open to the entire inlet chamber. The present invention discloses a particular form of structure and operation of such cartridge filters which has been found particularly satisfactory. Accordingly, it has been found that when a percentage of the inlet flow is taken across the inlet side of the filter cartridge in a tangential pattern and recycled to the input the commonly observed rapid filter element plugging and caking on the inlet side is greatly reduced. In many cases this reduction is sufficient to cause the filter to operate in nearly steady filtering state mode for a time very long in comparison with the time that the filter would operate normally in dead-ended configuration.